Optical element, and manufacturing method thereof

ABSTRACT

An optical element according to the present invention has a thin film, in which single-wall carbon nanotubes are laminated, and utilizes a saturable absorption function of the single-wall carbon nanotubes. Further, in a method for producing the optical element according to the present invention, the thin film is formed by spraying, to a body to be coated, a dispersion liquid prepared by dispersing the single-wall carbon nanotubes in a dispersion medium. Accordingly, a nonlinear optical element, which can operate in an optical communication wavelength region and which is extremely inexpensive and efficient, and a method for producing the optical element can be provided.

TECHNICAL FIELD

The present invention relates to an optical element (an optical device),which can control light in an optical communication wavelength region byutilizing a saturable absorption function of single-wall carbonnanotubes, and to a method for producing the optical element.

BACKGROUND ART

A carbon nanotube discovered recently is a tubular material, which isideally formed with a sheet structure of a hexagonal carbon lattice (agraphene sheet) parallel to an axis of the tube. Further, the carbonnanotube may be a multi-layered tube formed with plural sheets mentionedabove. In theory, the carbon nanotube is expected to exhibit a metallicor semiconducting property depending upon the connection type of thehexagonal carbon lattice and the diameter of the tube, and is expectedas a future functional material.

A material having a diameter of 1 μm or less, which is thinner than acarbon fiber, is commonly referred to as a carbon nanotube, and isthereby distinguished from the carbon fiber. However, there is nospecifically clear border therebetween. In a restricted meaning, a tubeformed with a graphene sheet of a hexagonal carbon lattice parallel toan axis of the tube is referred to as a carbon nanotube. (Thisrestricted meaning is applied to a carbon nanotube in the presentinvention.)

In general, the carbon nanotube defined by the restricted meaning isfurther classified. A tube formed with a sheet of a hexagonal carbonlattice is referred to as a single-wall carbon nanotube (hereinafter,sometimes simply referred to as an “SWNT”), and a multi-layered tubeformed with plural sheets of a hexagonal carbon lattice is referred toas a multiwall carbon nanotube (hereinafter, sometimes simply referredto as an “MWNT”). The method and conditions of synthesis determine, tosome extent, the structure of the carbon nanotube to be obtained.

In particular, the SWNT has attractive diversity exhibiting a metallicor semiconducting property in accordance with a chiral vector, and thushas been principally considered to be applied to an electric andelectronic element (see, for example, “Basics of Carbon Nanotube”authored by Yahachi Saito and Syunji Bando (1998), Corona PublishingCo., Ltd.). An attempt to improve the property of a field emittingelement by utilizing the efficient field electron emission property isin the advanced stage (see, for example, K. Matsumoto et al., ExtendedAbstracts of the 2000 International Conference on Solid State Devicesand Materials (2000), pp. 100-101). However, the SWNT has not beensufficiently studied so far with respect to optical applications.

In the case of application to an electric and electronic element, aminute probe can access a single carbon nanotube. On the other hand, inthe case of optical application, access is principally made to bulkcarbon nanotubes by using a luminous flux condensed to a diameter ofseveral hundreds of nm to several tens of μm. The primary reasons forthe delay in optical application of the SWNT as compared with electricand electronic element applications may be due to the difficulty inobtaining a high purity sample of the SWNT required for opticalevaluation, and the difficulty in forming an optically uniform film asthe SWNT is hard to dissolve in solvents. There is a report ofevaluation of a nonlinear optical constant aimed at optical applicationof the SWNT. However, in this report, an SWNT in a solution state isevaluated at 1,064 nm, 532 nm and 820 nm, which are not in a resonantregion, and there is no report of remarkable nonlinearity that promisespractical use (X. Liu et al., Appl. Phys. Lett. 74 (1999), pp. 164-166;Z. Shi et al., Chem. Commun. (2000), pp. 461-462).

The SWNT is known to have absorption at a wavelength of 1.8 μm, which isin an optical communication wavelength region (1.2 to 2 μm) (H. Katauraet al., Synth. Met. 103 (1999), pp. 2555-2558). If the resonant effectin this absorption band can be directly utilized, remarkablenonlinearity can be realized in a band of these wavelengths.

On the basis of the above-described consideration, we have studiedapplication of the SWNT to an optical element operating in an opticalcommunication wavelength region.

DISCLOSURE OF THE INVENTION

In order to solve the above-described problems, an optical element (anoptical device) according to the present invention has a thin film, inwhich single-wall carbon nanotubes are laminated, and utilizes asaturable absorption function of the single-wall carbon nanotubes.

In a method for producing the optical element (the optical device)according to the present invention, the thin film is formed by spraying,to a body to be coated, a dispersion liquid prepared by dispersing thesingle-wall carbon nanotubes in a dispersion medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the absorption property of an SWNT thin filmin the infrared region, wherein the optical energy irradiated onto theSWNT thin film is plotted in the abscissa axis and the absorbance of theSWNT thin film is plotted in the ordinate axis.

FIG. 2 is a graph, wherein an absorption band with the lowest energy isextracted from FIG. 1 and the optical energy in the abscissa axis ofFIG. 1 is replaced by the optical wavelength.

FIG. 3 is a schematic setup for explaining a Z-scan method.

FIG. 4 is a graph showing results of saturable absorption of the SWNTthin film measured by the Z-scan method.

FIG. 5 is a schematic setup showing outline of an experiment in whichthe optical element of the present invention is operated as a variabletransmittance type optical switch.

FIG. 6 is a schematic cross-sectional view showing an implementation ofan optical element having a function of a saturable absorption mirror.

FIG. 7 is a graph for explaining the principle of waveform shaping in acase where the optical element of the present invention is used as awaveform shaper having a function of waveform shaping, wherein time isplotted in the abscissa axis and the optical intensity of an incidentlight pulse is plotted in the ordinate axis.

FIG. 8 is a schematic cross-sectional view showing an implementation ofan optical element having a function of an ultra-high resolution opticaldisk.

MOST PREFERRED EMBODIMENTS FOR IMPLEMENTING THE INVENTION

An optical element and a method for producing the same of the presentinvention will be described in detail below.

Details of Optical Element and Method for Producing the Same of thePresent Invention

Carbon nanotubes include a single-wall carbon nanotube having astructure of a tube formed with a sheet of a hexagonal carbon lattice,and a multiwall carbon nanotube having a structure of a multi-layered(multi-walled) tube formed with plural carbon sheets mentioned above.The present invention employs single-wall carbon nanotubes that have ahigh saturable absorption function.

SWNTs having a diameter of 1.0 to 1.6 nm are preferably used. When theSWNTs having a diameter within the above range are used, the saturableabsorption function is effectively exhibited.

An optical element of the present invention utilizes optical absorptionof quasi-one-dimensional exciton caused by interband transition, whichoriginates in one-dimensional van Hove singularity of SWNTs in a 1.5 μmband. The wavelength of this absorption largely varies depending uponthe diameter of the SWNT. This is because the energy gap of the SWNT isproportional to a reciprocal of the diameter.

Background absorption other than the above-described absorption of theSWNTs in the 1.5 μm band is not so large, and thus even if many kinds ofSWNTs coexist, the function can be exhibited. This is because, when acertain number of SWNTs providing the absorption with the desiredwavelength exist, suitable optical absorption can be obtained, and theother SWNTs do not have a major effect on the absorption. However, ifthe SWNTs have an extremely wide diameter distribution, the opticalabsorption due to the undesired SWNTs (this absorption corresponds totail of π-plasmon absorption in ultraviolet region and thus has noeffect on the saturable absorption) prevails, and thereby theperformance of the obtained optical element may be degradedconsiderably. Accordingly, the SWNTs to be used preferably should haveas sharp a diameter distribution as possible which is centered at thediameter of tubes that provide the absorption at the desired wavelength.

A method for producing the SWNTs is not particularly limited, and anyconventionally known production methods, such as a thermal decompositionmethod using a catalyst (a similar method to a vapor growth method), anarc discharge method, and a laser ablation method, may be used. Asdescribed above, the SWNTs having a sharp diameter distribution arepreferably used in the present invention. At present, it is the laserablation method and the arc discharge method that realize the sharpdiameter distribution. However, SWNTs produced by the arc dischargemethod contain much catalytic metal (which obviously does not contributeto the performance of the element), and it is thus difficult to purifythe SWNTs to high purity.

Accordingly, SWNTs produced by the laser ablation method are preferablyused in the present invention. However, there is no problem in usingSWNTs produced by a CVD method or the like, as long as the SWNTs areuniform in diameter.

A process for producing, by the laser ablation method, the single-wallcarbon nanotubes suitable for the present invention will be exemplifiedbelow.

A rod made of mixture of graphite powder and fine powder of nickel andcobalt (mixing ratio: 0.45% in molar ratio, respectively) is prepared asa raw material. The rod is heated to 1250° C. in an electric furnaceunder 665 hPa (500 Torr) of argon atmosphere, and 350 mJ/pulse of secondharmonic pulse of Nd:YAG laser is then irradiated thereto for vaporizingcarbon and metallic fine particles, so as to produce single-wall carbonnanotubes (producing operation A).

The above-described production process is only a typical example, andthe type of the metal, the type of the gas, the temperature of theelectric furnace, the wavelength of the laser, and the like may bechanged. Further, single-wall carbon nanotubes produced by methods otherthan the laser ablation method, for example, a CVD (chemical vapordeposition) method, an arc discharge method, a method of thermaldecomposition of carbon monoxide, a template method in which organicmolecules are inserted into minute holes so as to be thermallydecomposed, and a fullerene-metal codeposition method, may be used.

Samples of the SWNTs after being produced by various methods inevitablycontain impurities to some extent, though the extent depends on theproduction method. In order to obtain an optical element with a goodperformance, the samples of the SWNTs are preferably purified.

A method of purification is not particularly limited. For example, thesamples of the SWNTs produced by the above-described laser ablationmethod using metallic fine particles of NiCo (producing operation A) canbe purified by the following procedure.

1. Heat Treatment in Vacuum

Heat treatment is carried out in vacuum in order to sublimate and removefullerene contained as impurities. In this heat treatment, a vacuumcondition is set around 10⁻¹⁴ Pa, and a temperature is set around 1250°C.

2. Washing with Toluene and Filtration

After the heat treatment in vacuum, washing is carried out with toluene.At the time of washing, the samples of the SWNTs are dispersed intoluene and stirred. Thereafter, filtration is carried out. At the timeof filtration, a mesh, which is sufficiently fine to filter the SWNTs,is used. (Such a mesh is also used for filtration in the followingsteps.)

3. Dispersion in Ethanol and Filtration

After the washing with toluene and the filtration, dispersion in ethanolis carried out as a pretreatment for producing pure water dispersionliquid. After the dispersion, filtration is carried out.

4. Dispersion in Pure Water

After the dispersion in ethanol and the filtration, the samples aredispersed in pure water to produce pure water dispersion liquid.

5. Addition of Hydrogen Peroxide Water

Hydrogen peroxide water is added to the obtained pure water dispersionliquid such that the amount of hydrogen peroxide is 15% (by volume) as awhole.

6. Reflux Operation and Filtration

In order to burn amorphous carbon contained as impurities, the purewater dispersion liquid, to which the hydrogen peroxide water has beenadded, is subjected to a reflux operation at 100° C. for 3 hours by areflux device. Thereafter, filtration is carried out.

7. Washing with Diluted Hydrochloric Acid and Filtration

After the reflux operation and the filtration, the samples are washedwith diluted hydrochloric acid to remove the metallic fine particles ofNiCo. At the time of washing, the samples of the SWNTs are dispersed indiluted hydrochloric acid and stirred. Thereafter, filtration is carriedout.

8. Washing with Aqueous Solution of Sodium Hydroxide and Filtration

After the washing with diluted hydrochloric acid and the filtration, thesamples are washed with an aqueous solution of sodium hydroxide for thepurposes of neutralizing the residual hydrochloric acid and removingby-product due to the acid treatment. At the time of washing, thesamples of the SWNTs are dispersed in an aqueous solution of sodiumhydroxide and stirred. Thereafter, filtration is carried out.

9. Kept in Vacuum at 650° C. for 1 Hour

After the washing with an aqueous solution of sodium hydroxide and thefiltration, the samples are kept in vacuum (around 10⁻⁴ Pa) at 650° C.for 1 hour for the purpose of removing various solvents contained in thesamples.

10. Cooling Down to Ordinary Temperature

The samples are then cooled down to an ordinary temperature so thatSWNTs having extremely high purity can be produced. (The above-describedsteps 1 to 10 are referred to as purification operation B.)

Incidentally, high purity SWNTs of no less than 90% and containing fewmetallic fine particles were obtained in the operation performed by thepresent inventors.

Any methods, by which the SWNTs can be purified to have such highpurity, can be used. For example, the SWNTs may be carefully heated inthe air to burn amorphous carbon, or the purification may be carried outusing diluted or concentrated nitric acid solution. The purification ofthe SWNTs to be used in the present invention may employ one of thesemethods.

An optical element of the present invention can be obtained by forming athin film in which the above-described SWNTs are laminated. A method forforming the thin film is not particularly limited as long as the thinfilm in which the SWNTs are laminated can be finally obtained. Examplesof the method include a spray method, an electrophoretic film-formingmethod, and a polymer dispersion method. The methods for forming thethin film will be described below.

Spray Method

The spray method is a method for forming the thin film by spraying adispersion liquid in which the SWNTs are dispersed in a dispersionmedium.

The purified SWNTs are dispersed in an appropriate dispersion medium toprepare a dispersion liquid. As the dispersion medium, alcohol,dichloroethane, dimethylformamide, or the like can be used.Dichloroethane and dimethylformamide are preferable in thatdispersibility is excellent and the quality of the thin film obtainedtherefrom is satisfactory. However, dichloroethane and dimethylformamidehave slightly low volatility, and thus, at the time of spray coating,which will be described later, some efforts are required. Examples ofsuch efforts include maintaining the body to be coated at hightemperature, and spending a longer period of time in forming the film byreducing the amount of the dispersion liquid to be sprayed. On the otherhand, alcohol is preferable in that volatility is high. Examples of thealcohol include methanol, ethanol, isopropyl alcohol (IPA) and n-propylalcohol, and among them, ethanol is particularly preferable.

At the time of preparing the dispersion liquid, if necessary, additivessuch as a surfactant can be used. Surfactants generally used asdispersants are preferably used. Preferable examples thereof include asurfactant having a polarity, and a surfactant having a functional groupwhich can easily bond with SWNTs chemically.

The concentration of the carbon nanotubes in the dispersion liquid isnot particularly limited. However, when ethanol is used as thedispersion medium, the concentration is preferably within a range of 1to 2 mg/ml.

After the SWNTs and the additive added as needed have been introduced inthe dispersion medium, it is desirable that the dispersion medium issufficiently stirred to disperse the SWNTs uniformly. A device used forstirring is not particularly limited. Examples thereof include ablade-type stirrer, a kneader, a roll mill and an ultrasonic disperser,and among them, an ultrasonic disperser is preferable.

The dispersion liquid obtained in the above-described manner is sprayedon a body to be coated. A method for spray coating is not particularlylimited, and the spray coating can be carried out with a known deviceand under known conditions, for example, with an airbrush. When theairbrush is used, it is effective to apply ultrasonic waves to theliquid container of the airbrush to disperse the SWNTs, which easilyaggregate, in the dispersion medium such as ethanol.

In the spray coating, if the temperature of the body to be coated islow, the dispersion medium does not quickly evaporate and thus the SWNTsform large aggregates on the surface of the body, and thereby thequality of the film may be degraded. Therefore, it is preferable thathot air is simultaneously blown over the body to be coated by a drier orthe body to be coated is directly heated by a heater so as to raise thetemperature of the body, such that the sprayed liquid evaporatesinstantaneously.

Electrophoretic Film-Forming Method

The purified SWNTs are dispersed in the same dispersion medium as in thespray method, for example, in dimethylformamide, with a concentrationaround 0.4 to 0.5 mg/ml, and 50 wt % sodium hydroxide aqueous solutionis added thereto in an amout of about 1% by mass (outer percentage).Then, a pair of electrodes are inserted in this dispersion liquid with adistance of about 1 cm there between, and a DC voltage is appliedbetween the electrodes. The voltage is preferably around 20 V. Theelectrical conduction therebetween allows for the SWNTs to migrate anddeposit on the surface of the positive electrode, so as to form a filmthereon. Namely, in this method, a body to be coated is the positiveelectrode.

Polymer Dispersion Method

The polymer dispersion method is a method in which the purified SWNTsare dispersed in an organic medium solution of a polymer such aspolystyrene and the dispersion liquid is applied on a surface of a bodyto be coated by an arbitrary coating means such as a spin coater. Thismethod provides a uniform film and thus is an effective method. However,this method is disadvantageous in that the chemical stability of theSWNTs may be degraded depending upon the polymer to be used.

As the polymer, any polymers that can form a film can be used. However,polystyrene or the like, which has little influence on the SWNTs, ispreferable. With respect to the organic medium, one that can dissolvethe polymer to be used should be appropriately selected. Theconcentration of the polymer in the organic medium solution should beappropriately adjusted in accordance with the coating ability. Further,the concentration of the SWNTs should be appropriately adjusted inaccordance with the desired amount of the SWNTs in the thin film.

Other Methods

It is also effective that a body such as a substrate to be coated isinserted into an apparatus for producing the SWNTs so that the SWNTs aredirectly trapped on the surface of the body. After forming a film, theamorphous carbon (i.e., impurities) is removed by an in-air oxidationmethod and the metallic catalyst is removed by an in-vacuo hightemperature heating and sublimating method. According to this method,the SWNTs can be purified so as to have sufficiently high purity, sothat a usable SWNT thin film can be obtained.

The SWNT thin film specific to the present invention can be formed bythe above-described methods. The amount of the SWNTs included in theSWNT thin film is determined such that the transmittance at the desiredwavelength is preferably around 0.1 to 10% and more preferably around 1%in order that the SWNT thin film exhibits a satisfactory saturableabsorption function.

Examples of a body, on which the SWNT thin film is formed, includesubstrates such as a glass substrate and a quartz substrate, opticalmaterials, and optical elements. When a body to be coated is asubstrate, an optical element utilizing the saturable absorptionfunction of the SWNT thin film itself can be produced. When a body to becoated is an optical material or an optical element, an optical elementutilizing the optical function of the body as well as the saturableabsorption function of the SWNT thin film formed can be produced.Specific examples of the body to be coated will be described later inembodiments.

The SWNT thin film obtained in the above-described way exhibits pluralabsorption bands in the infrared region. An absorption band with thelowest energy positions near a band of 1.2 to 2 μm which is an opticalcommunication wavelength region, and the absorption peak wavelength isaround 1.78 μm. Thus, an optical element, in which the SWNT thin film isformed, utilizes the saturable absorption function of the film so as tooperate in the communication wavelength region.

In this case where the SWNT thin film is used as a saturable absorptionmaterial in the communication wavelength region, the optical element isconsidered to have the following characteristics as compared with a caseusing a semiconducting material.

Firstly, the cost of the semiconductor device can be kept extremely low.Raw material of the SWNT is less expensive than a semiconductingmaterial, and the SWNT can be mass-produced. Additionally, since theSWNT does not require a process for forming a quantum structure such asa semiconducting quantum well, which is by a vacuum process, but merelyrequires that a thin film should be directly formed on a surface of abody such as a substrate to be coated, the SWNT can be easily producedand has a good yield. Accordingly, it is expected that an opticalelement can be produced at a lower cost by several orders of magnitudeas compared with a case using a semiconducting material.

Secondly, the SWNT thin film is formed in an existing optical element onthe manufacturing sites so that the saturable absorption function can beeasily imparted to the optical element. For example, by forming a SWNTthin film on a surface of a reflecting mirror (a body having a mirrorsurface), it is easy to produce a mirror having a reflectance that canbe varied by the intensity of incident light. In a case using aconventional semiconducting material, it has been required that aquantum well layer is directly formed on the reflecting mirror in avacuum process. According to the present invention wherein the SWNT thinfilm is merely formed, the production cost of an existing opticalelement can be considerably reduced. Furthermore, the saturableabsorption film can be formed at portions where it has been unable toform the film, and as a result, an unprecedented optical element may beproduced.

Thirdly, a thin film with a large area can be easily obtained. If oneattempts to form a thin film having the same saturable absorptionfunction by using a conventional semiconducting material, though thefilm can be enlarged to some extent, the production cost thereof mayincrease extremely, since a larger vacuum device is required. On theother hand, in the case of the SWNT thin film, since the film can bethinned by a simple coating method such as a spray coating, the obtainedarea of the film is not limited and the operation itself for forming thefilm is easy.

Fourthly, the material is expected to have an extremely high durabilityand light resistance. This is because the SWNT is structured only withrigid bonds called as sp2 conjugated bonds of carbon atoms, and theelectric conductivity is high and heat tends not to accumulate.

Further, since the SWNT is stable in the air and does not burn up toabout 500° C., the SWNT can be used at a high temperature in the air. Invacuum, the structure of the SWNT does not change up to 1600° C., theSWNT can be used at an even higher temperature.

Verification of Saturable Absorption Function of SWNT Thin Film

The saturable absorption function of the SWNT thin film formed in thepresent invention was verified in the following manner by actuallyforming a SWNT thin film.

SWNTs to be used were produced by the above-described laser ablationmethod using metallic fine particles of NiCo (producing operation A) andpurified by the above-described purification operation B.

A liquid, in which 1 to 2 mg of the above SWNTs were dispersed in 5 mlof ethanol by an ultrasonic disperser, was sprayed on a surface of aquartz substrate to form the SWNT thin film. At this time, hot air wassimultaneously blown over the quartz substrate by a drier to increasethe temperature of the quartz substrate, such that the sprayed liquidevaporated instantaneously.

The obtained SWNT thin film was black. As shown in the graph of FIG. 1,wherein the optical energy irradiated onto the SWNT thin film is plottedin the abscissa axis and the absorbance of the SWNT thin film is plottedin the ordinate axis, the SWNT thin film exhibited plural absorptionbands in the infrared region. FIG. 2 shows a graph, wherein anabsorption band with the lowest energy is extracted from FIG. 1 and theoptical energy in the abscissa axis of FIG. 1 is replaced by the opticalwavelength. As shown in FIG. 2, the absorption band with the lowestenergy positioned near a band of 1.5 to 2 μm, and the absorption peakwavelength was 1.78 μm. Raman spectrum and STM observation suggest thatthe diameters of the SWNTs distribute within a range of 1.2 to 1.6 nm.

The saturable absorption function of the SWNT thin film was measured bya so-called Z-scan method. The saturable absorption is a kind of thirdorder nonlinear optical effect, and is a phenomenon in which manyelectrons are excited to an upper level upon high intensity irradiationof laser beams corresponding to the absorption wavelength and theelectronic excitation is suppressed under such a condition, leading to atemporary decrease of absorption.

FIG. 3 is a schematic setup for explaining the Z-scan method. In theZ-scan method, laser beams L are emitted into a lens 3 via filters suchas a UV blocking filter 1 and an ND filter 2, and condensed to anintermediate point (focus X) between the lens 3 and an optical receiver5. Then, a sample 4 to be measured is moved from the lens 3 side to theoptical receiver 5 side along the direction in which the laser beams Lproceed. With respect to the position Z of the sample 4, the focus X isset 0 (zero), the position near the lens 3 from the focus X isrepresented by − (minus), and the position near the optical receiver 5from the focus X is represented by + (plus). In this way, the intensityof light irradiated onto the sample 4 is maximized when Z is 0, and isgradually reduced when the position moves apart towards the + or −direction. In other words, by merely moving the position Z of the sample4, the variation in the transmittance depending upon the intensity oflight irradiated onto the sample 4 can be measured by the opticalreceiver 5.

The reduction of the absorbance based on the absorption saturation wasestimated from the increase of the transmittance near the focus X byusing the obtained SWNT thin film (on the quartz substrate) as thesample 4. A femtosecond laser was used for a laser beam source, and thewavelength to be measured was set to 1.78 μm, which is the absorptionpeak of the SWNT thin film, by an optical parametric amplifier (OPA).

FIG. 4 shows a graph of measurement results. In FIG. 4, the abscissaaxis shows the position of the sample (Z), and the ordinate axis showsthe transmittance (ΔT/T) normalized by defining the transmittanceobtained when Z is −25 (mm) as 1. The increase of the transmittancecaused by the absorption variation was observed around the position Z=0(focus), and it was found that the SWNT thin film causes absorptionsaturation with absorption bands in the infrared region.

Assuming the thickness of the SWNT is 100 nm, the nonlinear opticalconstant is estimated to be about 10⁻⁶ esu in view of the intensity ofincident light. This value is smaller than that of a semiconductingquantum well (QW), which is a primary material for an optical switchingelement at present, by just a single order. Therefore, as a performanceindex, this value holds great promise as a material that can be easilyformed into a thin film from a dispersion liquid state. Incidentally,the nonlinear optical constant of phthalocyanine, which can be easilyformed into a thin film in the same manner as in the SWNT and which isknown as an organic nonlinear optical material having a highnonlinearity, is 10⁻¹⁰ to 10⁻¹² esu. Accordingly, it was verified thatthe SWNT holds great promise as a saturable absorption material in theinfrared region.

Embodiments of Optical Element of the Present Invention

Now, the optical element of the present invention will be explained withseveral preferable embodiments.

(1) Optical Switch

In the same manner as in the method described in the above section“Verification of saturable absorption function of SWNT thin film”, theSWNT thin film was formed on a glass substrate, so as to produce anoptical element exhibiting an optical switching operation. Theabsorption peak wavelength was 1.78 μm, and the absorbance was 1.3.

The obtained optical element was operated as a variable transmittancetype optical switch. The outline of this experiment is shown in FIG. 5.In FIG. 5, reference numeral 10 indicates an optical element having anoptical switching function, and the optical element 10 is structured byforming an SWNT thin film 11 on a surface of a glass substrate 12. Thewavelengths of controlling light and signal light were both set to 1.78μm, and the variation in quantity of transmitted light due to thepresence or absence of the controlling light was measured by a powermeter serving as an optical receiver 5. The controlling light and thesignal light were produced by converting the wavelength of femtosecondlaser beams with an OPA. The pulse width was 200 fs, and the cycleperiod was 1 kHz. As a result of the experiment, when the opticalintensity of the controlling light was 0.36 mJ/cm²·pulse, it wasobserved that the quantity of transmitted light increased by 60%. Thisverified that the SWNT thin film 11 served as an optical switchutilizing the saturable absorption. Accordingly, the optical element inthis embodiment can be used as an optical element (optical switch)having the optical switching function in a band of 1.2 to 2.0 μm.

(2) Saturable Absorption Mirror

As shown in FIG. 6, in the same manner as in the method described in theabove section “Verification of saturable absorption function of SWNTthin film”, an SWNT thin film 11 was formed on a surface of an Ag-coatmirror (optical element), in which a surface of a glass substrate 12 wascoated with a silver (Ag) mirror layer 14, so as to produce an opticalelement having a function of a saturable absorption mirror.

With respect to the obtained optical element, dependence of thereflecting light intensity on the irradiating light intensity wasmeasured. As a result, with irradiation of laser beams having awavelength of 1.78 μm and a pulse width of 200 fs, an increase in thereflecting light intensity was observed when the irradiating lightintensity more or less exceeds 10 μJ/cm²·pulse. When the irradiatinglight intensity was 300 μJ/cm²·pulse, the reflecting light intensity wasapproximately doubled as compared with that obtained when theirradiating light intensity was 10 μJ/cm²·pulse. This verified that theAg-coat mirror, on which the SWNT thin film 11 was formed, served as asaturable absorption mirror. Accordingly, the optical element in thisembodiment can be used as an optical element having the function of thesaturable absorption mirror in a band of 1.2 to 2.0 μm.

(3) Waveform Shaper

When the SWNT thin film having the saturable absorption function isutilized, an optical element having a function of waveform shaping,e.g., shortening a time width of an incident light pulse, can be formed.The structure of the optical element is basically the same as in theabove section “(1) Optical switch”.

FIG. 7 shows a graph for explaining the principle of waveform shaping inthe case where the optical element of the present invention is used as awaveform shaper having a function of waveform shaping. In the graph ofFIG. 7, the abscissa axis shows time, and the ordinate axis shows theoptical intensity of an incident light pulse. On the time axis, theoptical element of the present invention has a low transmittance nearboth ends of the pulse where the optical intensity is low, and has ahigh transmittance near a center portion of the pulse where the opticalintensity is high. As a result, the both ends of the pulse that hastransmitted through the SWNT thin film are cut (or reduced) so that thepulse has a shorter time width than the original pulse.

A waveform shaping experiment was carried out by an infrared OPA system.As laser beams in the infrared OPA system, infrared light having a pulsewidth of 4 to 6 ns and a cycle period of 10 Hz can be lased. In order toobtain a satisfactory peak optical intensity, measurement was carriedout by condensing output light having a wavelength of 1.78 μm and anoptical intensity of 3 mW into 50 μmφ on the SWNT thin film. As a resultof observing, with a photodetector, the time width of the light whichhad transmitted through the SWNT thin film, the time width was shortenedby about 30% than the original pulse. This verified that the SWNT thinfilm served as a waveform shaper. Accordingly, the optical element inthis embodiment can be used as an optical element having the function ofwaveform shaping in a band of 1.2 to 2.0 μm.

(4) Ultra-High Resolution Optical Disk

In the case where the optical element of the present invention is usedas the waveform shaper, the pulse width on the time axis can beshortened. Now, in a case where the optical element of the presentinvention is used as an ultra-high resolution optical disk by utilizingthe same saturable absorption function, a spatial beam diameter can bereduced.

FIG. 8 is a schematic cross-sectional view showing an optical element ofthe present invention having a function of an ultra-high resolutionoptical disk, which reduces a spatial beam diameter to realizeultra-high resolution. In FIG. 8, pits 23 are formed in one side surfaceof a substrate 21, which is made of a plastic material such as apolycarbonate resin, an acrylic resin, or a polyolefin resin. Then, areflective layer 22, which is made of a metal such as gold, silver,aluminum, platinum or copper, or of an alloy containing these metals, isprovided thereon (not in the gravitational direction but in thelaminating direction), and a protective layer 24 is further providedthereon (in the laminating direction). The substrate 21, the reflectivelayer 22 and the protective layer 24 form an optical disk 25, and anSWNT thin film 11 is formed on a surface of the optical disk 25 on thesubstrate 21 side. (Hereinafter, this surface of the optical disk 25 isreferred to as a “recording surface”. In the present invention, the“recording surface” is a surface into which irradiation beams areemitted.) The layer structure of the optical disk 25 is not limited tothat shown in FIG. 8.

Since laser beams have a Gauss type beam pattern, the optical intensityat a center portion thereof is higher than that at a peripheral portionthereof. Therefore, if the SWNT thin film 11 is formed on the recordingsurface of the optical disk 25, when laser beams are irradiated from therecording surface side, only a part of a center portion of theirradiation beams transmits due to the saturable absorption function ofthe SWNT thin film 11. This effect enables a spot, which is smaller thana condensing limit of the beams, to be formed on the recording surfaceof the optical disk 25. Such an optical disk having this function isreferred to as an ultra-high resolution optical disk.

With respect to the optical element shown in FIG. 8 (ultra-highresolution optical disk) in which the SWNT thin film 11 was formed inthe same manner as in the method described in the above section“Verification of saturable absorption function of SWNT thin film”, theeffect of the reduced beam diameter was observed. The infrared OPAsystem, which was used for the above-described waveform shapingexperiment, was used for irradiation of laser beams. Output light havinga wavelength of 1.78 μm was condensed onto a surface of the SWNT thinfilm 11, observed by a beam profiler, and compared to the original beamdiameter. As a result, the beam pattern changed and the luminance onlyat the center portion of the beams increased selectively when theirradiating light intensity more or less exceeded 1 mW. Under optimalconditions (about 3 mW), the radius of the beam diameter could bereduced to about 60% of the original one. This verified that the SWNTthin film served as a material for an ultra-high resolution opticaldisk. Accordingly, the optical element in this embodiment can be used asan optical element having the function of the ultra-high resolutionoptical disk.

In this way, according to the structure of the present inventionutilizing the saturable absorption function of the SWNT thin film, anactive element such as the optical switch, in which signal light can beactively controlled by extraneous controlling light, and a passiveelement such as the saturable absorption mirror, in which signal lightis passively controlled by the signal light itself, can be producedarbitrarily.

Four embodiments of the optical element of the present inventionutilizing the saturable absorption function of the SWNT thin film havebeen described above. However, the present invention is not limited tothe embodiments described above, and is generally applicable to opticalelements produced by utilizing the saturable absorption function due toresonant excitation of the SWNT thin film.

Industrial Applicability

As described above, according to the present invention, the SWNT isapplied to an optical element so that a nonlinear optical element, whichcan operate in an optical communication wavelength region and which isextremely inexpensive and efficient, and a method for producing theoptical element can be provided.

1. An optical element comprising a thin film, in which single-wallcarbon nanotubes are laminated, and utilizing a saturable absorptionfunction of the single-wall carbon nanotubes.
 2. The optical element ofclaim 1, wherein the thin film is formed on a surface of a substrate. 3.The optical element according to claim 1, wherein the thin film isformed on a surface of an optical material or on a surface of an opticalelement.
 4. The optical element according to claim 1, wherein the thinfilm is formed by spraying a dispersion liquid in which the single-wallcarbon nanotubes are dispersed in a dispersion medium.
 5. The opticalelement according to claim 4, wherein the dispersion medium is alcohol.6. The optical element according to claim 1, utilizing saturableabsorption of the single-wall carbon nanotubes in a band of 1.2 to 2.0μm.
 7. The optical element according to claim 6, wherein each of thesingle-wall carbon nanotubes has a diameter of 1.0 to 1.6 nm.
 8. Theoptical element according to claim 1, exhibiting an optical switchingoperation due to transmittance variation caused by saturable absorptionof the thin film.
 9. The optical element according to claim 8, having anoptical switching function in a band of 1.2 to 2.0 μm.
 10. The opticalelement according to claim 1, wherein the thin film is formed on amirror surface so that the optical element has a function of a saturableabsorption mirror.
 11. The optical element according to claim 10, havingthe function of the saturable absorption mirror in a band of 1.2 to 2.0μm.
 12. The optical element according to claim 1, having a function ofwaveform shaping.
 13. The optical element according to claim 12, havingthe function of waveform shaping in a band of 1.2 to 2.0 μm.
 14. Theoptical element according to claim 1, wherein the thin film is formed ona recording surface of an optical disk so that the optical element has afunction of an ultra-high resolution optical disk.
 15. A method forproducing an optical element comprising a thin film, in whichsingle-wall carbon nanotubes are laminated, and utilizing a saturableabsorption function of the single-wall carbon nanotubes, wherein thethin film is formed by spraying, to a body to be coated, a dispersionliquid prepared by dispersing the single-wall carbon nanotubes in adispersion medium.
 16. The method for producing an optical elementaccording to claim 15, wherein the body to be coated is a substrate. 17.The method for producing an optical element according to claim 15,wherein the body to be coated is an optical material or an opticalelement.
 18. The method for producing an optical element according toclaim 15, wherein the dispersion medium is alcohol.
 19. The method forproducing an optical element according to claim 15, wherein each of thesingle-wall carbon nanotubes has a diameter of 1.0 to 1.6 nm.